The present invention relates to a control for a hydraulic unit. More particularly, this invention relates to a hybrid displacement control device which provides both direct and servo assisted control of hydraulic units. Such a control mechanism is useful in a variety of hydraulic applications, including but not limited to the propulsion system of a skid steer loader.
Hydraulic units, such as hydrostatic pumps and motors, typically have either a direct manual control mechanism or a servo displacement control mechanism. One advantage of direct manual displacement controls is that they are usually quite simple. They comprise a lever attached directly to the swashplate. The machine operator moves the lever directly or indirectly with cable or linkage means. The operator must supply all of the physical effort or force to stroke the hydrostatic unit. As a result, the operator directly "feels" the load on the hydrostatic system and the position of the control. This load and position feedback is desirable in certain applications, such as vehicle propulsion systems, especially when the operator desires to make precise movements with the vehicle. The load feedback feature, however, requires the operator to supply increased physical force as the load increases. This can result in operator fatigue. However, direct control devices typically have a smaller "dead band" than servo control mechanisms.
A servo displacement control typically utilizes a pressurized hydraulic cylinder to move the swashplate, thereby stroking the hydrostatic unit. Servo control devices provide hydraulic leverage so that a very small degree of physical effort is required to actuate the hydrostatic unit. With this type of control it is even possible to utilize an electro-hydraulic mechanism to stroke the hydrostatic unit with an electrical signal. Additionally, servo controls eliminate the transmission of vibration from the hydrostatic unit through the operator controls. Servo controls reduce the problem of operator fatigue, but load and position feedback and operator "feel" are sacrificed.
Thus, there is a need for a hybrid control which provides servo assisted control at high load conditions and direct manual control at low load conditions to reduce the stroking force required without totally sacrificing operator "feel" under low load conditions.
Therefore, a primary objective of the present invention is the provision of an improved control for hydraulic units.
A further objective of the present invention is the provision of a servo type control which also incorporates a direct displacement control mechanism so as to enable the hydraulic unit to be stroked manually under low load conditions to provide the desired operator "feel".
A further objective of the present invention is the provision of a control which avoids undesirable operator fatigue at high load conditions.
A further objective of the present invention is the provision of a servo type displacement control that provides load and position sensitive feedback to the machine operator.
A further objective of the present invention is the provision of a control which functions as a stroke assist device to lessen the operator effort required to actuate the hydraulic unit at conditions of increased load.
A further objective of the present invention is the provision of a control which simultaneously achieves both of the last two objectives mentioned above.
A further objective of the present invention is the provision of a bias coupling which includes a spring mechanism between the control spool and the control sleeve.
A further objective of the present invention is the provision of a control which is economical to produce, comfortable to use, non-fatiguing and reliable.
These and other objectives will be apparent in view of the drawings, as well as the description and the claims which follow.